The present invention relates to a relief valve. More particularly, the present invention relates to a combination pressure and thermally activated relief valve for use with a compressed gas storage cylinder (or pressure vessel).
It is desirable for some compressed gas storage cylinders to have safety valves such as a thermal relief device (TRD) and an over pressure relief device (PRD). For example, cylinders which hold compressed natural gas are required by law to have such relief valves.
In the past, such cylinders have used thermal relief valves which are activated when the body of the thermal relief valve exceeds a temperature threshold. For example, thermal relief valves of the prior art would typically have a passage which communicated with the interior of the cylinder. However, the passage would be filled (blocked) with a eutectic metal slug which remained solid below the predetermined melting temperature of the eutectic. Then, if the temperature of eutectic substance raised above the predetermined melting temperature, the slug would soften or melt. In its softened or weakened state, the eutectic substance would be forced out of the passage in the relief valve by the pressure of the gas in the cylinder. With the eutectic substance removed from the passage, the gas would be released through the passage to the external atmosphere or another suitable container.
However, such a relief valve was plagued by certain problems. The problems primarily resulted because the eutectic metal was exposed to the gas pressure in the cylinder. Under such pressure, the eutectic substance underwent what is known as plastic flow or creep. In other words, the gas pressure acting on the eutectic block over time would push some of the eutectic block out of the passage. Thus, even though no over-temperature condition existed, the thermal relief valve would be partially activated and leak. Since the eutectic substance, exposed to the gas pressure, was prone to plastic flow or creep, the life expectancy of such a thermal relief valve was relatively short. Thus, such relief valves need to be replaced more often than desirable.
Pressure relief devices have typically used a thin metal disc or wafer to block the flow passage. This disc simply ruptures when the gas pressure in the cylinder exceed a predetermined level.
Since pressure and thermal relief devices are typically incorporated into the cylinder shut off valve, it is desirable from cost and convenience standpoint to combine them.
Combination pressure relief and thermal relief devices have typically used a eutectic metal "slug" backed by a thin metal disc. Although this design solves the eutectic creep problem, it obviously requires a minimum gas pressure to rupture the disc, and has thus proven to be inoperative on partially filled cylinders. A letter warning against the use of this type of relief device was circulated by the United States and Canadian Gas Associations several years ago.
Since the eutectic creep problem is diminished with the use of smaller and/or partially restricted bores/openings, one solution is to simply limit the physical size of the TRD. While this is a perfectly practical solution for small volume cylinders, it is totally impractical for cylinders used on large trucks or buses. A recent CNG Urban Bus demonstration project utilizing three (3) long storage cylinders mounted on the bus roof required a total of twenty-seven (27) TRDs to meet the required emergency flow rate.